This invention relates to a device such as an adjacent channel interference canceller for processing a device input signal carrying a digital symbol sequence timed by a reference clock signal to produce a device output signal to a demodulator. The demodulator demodulates the device output signal into a reproduced clock signal, a reproduced symbol sequence, and a digital error signal. The reproduced clock signal is a reproduction of the reference clock signal. The reproduced symbol sequence is a reproduction of the digital symbol sequence.
In general, a device of the type described is supplied with a device input signal carrying the digital symbol sequence. The device comprises a reception filter for filtering the device input signal to produce a filter output signal. The filter output signal has an amplitude distortion resulting in a desired frequency band from adjacent interference on the desired frequency band by one or two frequency bands adjacent to the desired frequency band. The device serves to equalize the amplitude distortion, as well known in the art.
A conventional device is disclosed in U.S. Pat. No. 4,333,063 issued to Toshihiko Ryu and assigned to the instant assignee. According to Ryu, the device comprises an adaptive amplitude equalizing unit responsive to the filter output signal for equalizing the amplitude distortion along a frequency axis of the filter output signal to produce an amplitude equalized signal as the device output signal.
The device input signal has an intermediate level at a center of the desired frequency band. The desired frequency band is adjacent to one or two other frequency bands at sides of the other frequency bands. The device input signal has side levels at the sides of the desired frequency band. Therefore, the device input signal inevitably includes the side levels as its level components in two regions of the desired frequency band that are adjacent to the other frequency bands.
It has now been found that intersymbol interference is produced in the amplitude equalized signal by over-suppression to the side levels on the Ryu's adaptive amplitude equalizing unit. This is because the side levels could not appropriately suppressed by the reception filter.
The device input signal has a decrease in the intermediate level when subjected to fading or the like. The side levels are usually lower than the intermediate level.
When the decrease occurs, the side levels are overestimated to be higher than the intermediate level. As a result, the adaptive amplitude equalizing unit operatively over-suppresses the side levels to produce the amplitude equalized signal accompanied by the intersymbol interference.